A plasma processing using plasma has been widely employed in a manufacturing process of semiconductors, liquid crystal devices, and the like. As shown in FIG. 8, the plasma processing apparatus for performing such a plasma processing includes, in a processing vessel 10 made of vacuum chamber, a mounting table 11 serving as a lower electrode for mounting thereon a substrate, e.g., a semiconductor wafer W; and a shower head 12 disposed above the mounting table 11 and provided with a number of gas supply holes 12A. An upper electrode 13 is provided on a lower surface of the shower head 12. In the plasma processing apparatus having the above configuration, by applying a plasma generation high frequency power to either one of the upper electrode 13 and the mounting table 11, e.g., to mounting table 11, with a high frequency power supply 14, a plasma is generated in a processing space between the mounting table 11 and the upper electrode 13, and the processing gas introduced into the processing vessel 10 through the shower head 12 is activated by the plasma, so as to perform a plasma processing, e.g., an etching or a film forming process, on the wafer W. Further, the high frequency power from the plasma in the processing space reaches the upper electrode 13 and flows therefrom to the ground via a wall portion of the processing vessel 10. In FIG. 8, reference numeral 17 is a gas exhaust path through which the atmosphere in the processing vessel 10 is exhausted to outside.
The upper electrode 13 is constructed with a metal base (base member) 15 such as aluminum (Al), stainless steel (SUS) or the like; and a conductive plate 16 such as a silicon (Si) plate, a silicon carbide (SiC) plate or the like, wherein the conductive plate 16 tightly fixed on the surface of the metal base by screws, clamps or the like. By adopting such configuration, the entire structure of the upper electrode 13 is prevented form being deformed by a stress exerted thereon by the depressurized atmosphere inside processing vessel 10; and parts of the upper electrode 13 exposed to the plasma can be of a structure having tolerance to plasma to prevent metal contamination.
The upper electrode 13 is heated to a high temperature by the plasma generated in the processing space. Since, however, Si or SiC has a thermal expansion coefficient (linear expansion coefficient) that is smaller than that of the metal base 15, a dimensional difference is produced therebetween when the upper electrode 13 is heated. As a result, an excessive tensile stress is exerted to a fixed portion of the conductive plate 16, which often results in a destruction of the conductive plate 16.
In order to solve the problem, it has been attempted to prevent a tensile stress from being applied to the conductive plate 16 by the thermal expansion of the metal base 15 by movably fixing the conductive plate 16 to the metal base 15 by means of screws or clamps, in consideration of the dimensional difference that would be produced therebetween due to thermal expansion.
However, in order to perform a plasma processing uniformly throughout on the entire surface of the wafer W, the concentration of plasma active species needs to be consistent throughout a plane parallel to the wafer W. To obtain the consistency, an electric and thermal state of the conductive plate 16 exposed to the plasma needs to be consistent throughout the entire surface thereof. Accordingly, the conductive plate 16 and the metal base 15 are required to be contacted to each other in a manner that an electric and thermal conduction and a heat transfer can be uniformly carried out through the entire contact surfaces thereof. In other words, the two components need to be connected while preserving a highly uniform in-plane contact state. In many cases, however, only peripherial portions of the conductive plate 16 are fixed to a metal base 15. Thus, in case of movably fixing the conductive plate 16, local irregularities occur in the upper electrode 13 in terms of the contact state between the metal base 15 and the conductive plate 16. As a result, it becomes difficult to obtain a high in-plane uniformity in the electric and thermal conduction between the metal base 15 and the conductive plate 16. Thus, it is very likely that the conductive plate 16 is subject to a breakage, a disordered process, an abnormal discharge or the like. Furthermore, the contact surfaces of the metal base 15 and the conductive plate 16 would be rubbed against each other due to a dimensional variation caused by a thermal expansion and contraction during a temperature increase and decrease cycle. Therefore, dusts could be generated at the gas discharge openings 12a. 
In the meanwhile, Patent Reference 1 discloses an upper electrode including a metal base and a dielectric, wherein the electrode is fabricated by cutting a central portion of a porous ceramic to have a vertical cross section of a trapezoid shape; when viewed from a longitudinal cross section, fitting the dielectric into the cutout portion; and then impregnating a metal into the porous ceramic to form the metal base. While the metal is impregnated, the metal base and the dielectric are joined to each other by the impregnated metal. According to this technique, a high frequency power at a central portion of the electrode can be reduced by the presence of the dielectric inside the central portion of a metal-ceramic composite material, so that the strength of an electric field can be made uniformly. However, Reference 1 is mute on how to connect the metal base and the conductive plate to obtain a uniform in-plane contact state in which a uniform electric and thermal conduction can be carried out therebetween.
Patent Reference 1: Japanese Patent Laid-open Application No. 2005-228973 (paragraphs 0032 and 0053-0055 and FIG. 9)